1. Field of the Invention
The invention disclosed broadly relates to electronic filtering devices, and more particularly relates to the design of filters using microstrip elements and utilizing parasitic and field effects.
2. Description of the Related Art
FIG. 1 shows a block diagram of prior art for a downconverter, which is a frequency converter that shifts an input frequency to a lower frequency. The control circuit 102 controls the signal generator 104, which may be a voltage controlled oscillator ("VCO"). The signal generator 104 produces a local oscillator signal (the "LO signal") that contains a specific and desired frequency (the "LO frequency"), as well as a series of harmonics at multiples of the LO frequency. An attenuator 106 is used to protect the signal generator 104 by dampening any signals that are reflected and travel back into it. In the circuit of FIG. 1, filter 108 filters the second harmonic, which has a frequency of twice the LO frequency, from the signal generator 104, and filter 112 filters the second harmonic generated by the non-linearities of the LO amplifier 110. Filters 108 and 112 are generically shown because they can be implemented in a variety of means, including low pass, bandpass, or notch. The capacitors 109 and 111 are used for direct current ("DC") decoupling from the bias currents of the LO amplifier 110.
The second harmonic is then further filtered by notch filter 114 before the LO signal is mixed with the input signal, which is often in either the radio frequency ("RF") or microwave range. Because this example is a downconverter, the frequency of interest at the output of the mixer 116 is the intermediate frequency ("IF"), and the filter 118 has therefore been chosen as a lowpass filter that isolates the IF range.
Notch filters 120 and 124 further reduce, both before and after the IF amplifier 122, any LO frequency that may still be present on the signal, and the capacitors 121 and 123 are again used for DC decoupling from the bias currents of the IF amplifier 122. The surface-acoustic wave ("SAW") filter 126 is a narrow bandpass filter that is used to further isolate the IF ranged. SAWs are acoustically coupled devices, however, and therefore they typically do not reject frequencies far removed from their passband. Because the LO frequency is far removed from the IF, the SAW filter 126 does not adequately filter it and the notch filters 120, 124 are necessary to remove the LO frequency.
Because of the number of filters required to accomplish the downconversion, these circuits require a large board. Additionally, filters must typically be tuned after manufacture to ensure that they filter the appropriate frequencies, and this slows the manufacturing process and makes it harder to achieve identical performance from each circuit. Accordingly, there is a need for filtering devices and methods of designing filters which overcome these problems.